While low-voltage thermostats are very reliable and rarely defective, thermostat-related problems, such as faulty wires or loose connections, may cause a failure in your heating or cooling system. The problems are easy to identify, and just as easy to fix by making the appropriate repair or by replacing a faulty thermostat.

Step 1

Turn off the power to heating and/or cooling systems at the main service panel or system shutoff switch.

Step 2

Remove the thermostat. On some models, remove the cover plate and then unscrew the thermostat body from a wired base plate secured to the wall. Other models plug into the base plate without screws.

Step 3

Remove any dust from the thermostat and base plate with a soft artist's brush.

Step 4

Check for broken, frayed or corroded wires and loose wire connections. Tighten any loose connections. Use wire cutters/strippers to cut damaged wires and strip off about 1/2 inch (12 mm) of insulation, then reconnect the wires.

Step 5

Restore the power to test the thermostat. Consult the manual for your thermostat and disconnect the power wire (usually red), then touch it to the terminal for the heat (the terminal marked as W with white wire from heater's transformer).

Step 6

Similarly bypass the thermostat to check its cooling function by touching the disconnected power wire (red) to the terminal for the cooling system (typically yellow).

Step 7

If one or both systems fail to start, turn off their power, then identify (see step 8) and replace any faulty wires between the thermostat and the low-voltage transformer(s) at the heating or air-conditioning units.

Step 8

One way to test thermostat wires--on your heating system, for example--is to disconnect the wires from the thermostat and transformer, wire them together at one end, and test for continuity at the other end by clipping a continuity tester to one wire and probing the other wire. If the tester fails to light, replace the wires.

Step 9

Check for loose low-voltage wire connections at the transformer or loose line-voltage connections to the transformer.

Please note: These "do-it-yourself" guidelines are provided as an overview, and should not be used as the sole instructions for repairing a thermostat installation. Always read and follow the manufacturer's directions.

National Trade Supply, LLC cannot be held responsible for injuries or damages resulting from these instructions.